Serpentine cooling passages inside a turbine blade are formed between external airfoil walls and internal partition walls. The external walls are in direct contact with hot combustion gases, and need sufficient cooling to maintain adequate material life. The interior surfaces of the external hot walls are the primary cooling surfaces. The internal partition walls are extensions from the hot walls, and have no direct contact with the hot gas, so they are much cooler. The surfaces of the internal partition walls serve as extended secondary cooling surfaces for the external hot walls by conduction. Cooling air flows through the serpentine cooling passages and picks up heat from the walls through forced convection. The effectiveness of this heat transfer rate is inversely proportional to the thermal boundary layer thickness. Turbulators are commonly cast on the interior surfaces of the hot external walls to promote flow turbulence and reduce the thickness of the thermal boundary layer for better convective heat transfer. High-temperature alloys generally have low thermal conductivity and therefore have low fin efficiency in heat transfer. To improve the internal cooling inside a turbine blade, it is important to have sufficient directly cooled primary surface with effective turbulators.
In a turbine blade, the airfoil typically has a larger thickness near the mid-chord region. In order to maintain sufficient speed of the cooling air inside cooling passages, the cooling passages near the maximum airfoil thickness location become very narrow, as shown in FIG. 3 passages 47 and 48. These narrow passages have small primary cooling surfaces on the hot walls, and large secondary cooling surfaces on the partition walls. The small primary cooling surfaces also limit the size of the turbulators and their effectiveness. These narrow passages cannot provide good convective cooling. The invention described herein significantly increases the primary cooling surfaces on the hot walls and provides sufficient surface area for effective turbulators.